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Line 174: − + − [[file:using-and-improving-surface-models-built-by-computer_fig18.png|thumb|300px|{{figure number|18}}The cell is centered on the grid node and lies either inside or outside the polygon. The cell's area is multiplied by its z value (thickness) and that volume is added to volumes for all other cells inside the polygon.]]+ + + + + + + + Line 190:
Line 197: − − [[file:using-and-improving-surface-models-built-by-computer_fig19.png|300px|thumb|{{figure number|19}}The cell's corners are defined by grid nodes. The top is defined by two or more planes passing through the node z values and lie inside the polygon. The prism of volume under each plane is calculated and added to volumes for all other prisms inside the polygon.]] − − [[file:using-and-improving-surface-models-built-by-computer_fig20.png|thumb|300px|{{figure number|20}}A mathematical surface is fit to the grid cell. Calculus is used to integrate the volume under the curve, inside the grid cell, and inside the polygon. All cell volumes inside the polygon are added together.]] Line 208:
Line 211: − − [[file:using-and-improving-surface-models-built-by-computer_fig21.png|300px|thumb|{{figure number|21}}Thickness Is normally defined by grids representing the top and base of reservoir and the fluid contact(s).]] − − [[file:using-and-improving-surface-models-built-by-computer_fig22.png|300px|thumb|{{figure number|22}}The gross hydrocarbon rock thickness is progressively reduced by net to gross ratio, average porosity, and oil saturation, until only the thickness of pores filled with hydrocarbon remains.]] Line 226:
Line 225: − − [[file:using-and-improving-surface-models-built-by-computer_fig23.png|thumb|300px|{{figure number|23}}The envelope technique is used to define one grid for the top of reservoir and another for the base of the reservoir. These are subtracted to create the gross hydrocarbon rock thickness.]] Line 236:
Line 233: − − [[file:using-and-improving-surface-models-built-by-computer_fig24.png|300px|thumb|{{figure number|24}}Incomplete data for net and porosity due to partial penetrations, truncations, baselaps, and so on create problems when building models of these and related variables.]]
Using and improving surface models built by computer (view source)
Revision as of 14:00, 8 July 2014
, 14:00, 8 July 2014→Volumetrics
==Volumetrics==
==Volumetrics==
<gallery mode=packed heights=200px widths=200px>
using-and-improving-surface-models-built-by-computer_fig18.png|{{figure number|18}}The cell is centered on the grid node and lies either inside or outside the polygon. The cell's area is multiplied by its z value (thickness) and that volume is added to volumes for all other cells inside the polygon.
using-and-improving-surface-models-built-by-computer_fig19.png|{{figure number|19}}The cell's corners are defined by grid nodes. The top is defined by two or more planes passing through the node z values and lie inside the polygon. The prism of volume under each plane is calculated and added to volumes for all other prisms inside the polygon.
using-and-improving-surface-models-built-by-computer_fig20.png|{{figure number|20}}A mathematical surface is fit to the grid cell. Calculus is used to integrate the volume under the curve, inside the grid cell, and inside the polygon. All cell volumes inside the polygon are added together.
using-and-improving-surface-models-built-by-computer_fig21.png|{{figure number|21}}Thickness Is normally defined by grids representing the top and base of reservoir and the fluid contact(s).]]
using-and-improving-surface-models-built-by-computer_fig22.png|{{figure number|22}}The gross hydrocarbon rock thickness is progressively reduced by net to gross ratio, average porosity, and oil saturation, until only the thickness of pores filled with hydrocarbon remains.
using-and-improving-surface-models-built-by-computer_fig23.png|{{figure number|23}}The envelope technique is used to define one grid for the top of reservoir and another for the base of the reservoir. These are subtracted to create the gross hydrocarbon rock thickness.
using-and-improving-surface-models-built-by-computer_fig24.png|{{figure number|24}}Incomplete data for net and porosity due to partial penetrations, truncations, baselaps, and so on create problems when building models of these and related variables.
</gallery>
Estimates of volumes are normally calculated between two structure surfaces, above a fluid contact, or sometimes below another fluid contact.
Estimates of volumes are normally calculated between two structure surfaces, above a fluid contact, or sometimes below another fluid contact.
This approach assumes that each grid cell extends from a base up to a flat top, which is the grid node's value, and that the node is in the center of the cell. If the center of the cell is inside the polygon, it is counted; if it is outside, it is not ([[:file:using-and-improving-surface-models-built-by-computer_fig18.png|Figure 18]]).
This approach assumes that each grid cell extends from a base up to a flat top, which is the grid node's value, and that the node is in the center of the cell. If the center of the cell is inside the polygon, it is counted; if it is outside, it is not ([[:file:using-and-improving-surface-models-built-by-computer_fig18.png|Figure 18]]).
====Volume by simple plane fits====
====Volume by simple plane fits====
Grid nodes occupy corners of grid cells, and a line is drawn diagonally across a cell dividing it into two triangles. The value at each corner of the triangle is known; therefore, a flat plane can be fit through these points. The base above which volumes are to be calculated is also a plane, and its value is known. Since the dimensions of the sides of the triangular prism are known from the grid increments, all of the information needed to calculate the prism's volume is available. The volume of all triangle prisms totally inside the polygon are calculated and summed. Any prisms partially within the polygon are subdivided into smaller prisms with the values at the corners of the smaller prisms linearly interpolated from the three original triangle corners. The volumes of these partial values are calculated, summed, and added to the volume of prisms totally within the polygon ([[:file:using-and-improving-surface-models-built-by-computer_fig19.png|Figure 19]]).
Grid nodes occupy corners of grid cells, and a line is drawn diagonally across a cell dividing it into two triangles. The value at each corner of the triangle is known; therefore, a flat plane can be fit through these points. The base above which volumes are to be calculated is also a plane, and its value is known. Since the dimensions of the sides of the triangular prism are known from the grid increments, all of the information needed to calculate the prism's volume is available. The volume of all triangle prisms totally inside the polygon are calculated and summed. Any prisms partially within the polygon are subdivided into smaller prisms with the values at the corners of the smaller prisms linearly interpolated from the three original triangle corners. The volumes of these partial values are calculated, summed, and added to the volume of prisms totally within the polygon ([[:file:using-and-improving-surface-models-built-by-computer_fig19.png|Figure 19]]).
====Volume by mathematical fit and integration====
====Volume by mathematical fit and integration====
Political boundaries, [[porosity]] cutoffs, or other types of constraints must also be incorporated into the calculation. In some systems, these are incorporated into the model before going into the volumetrics program. In others, they are handeled automatically by the program.
Political boundaries, [[porosity]] cutoffs, or other types of constraints must also be incorporated into the calculation. In some systems, these are incorporated into the model before going into the volumetrics program. In others, they are handeled automatically by the program.
===Modeling and volume calculation in a thickness domain===
===Modeling and volume calculation in a thickness domain===
There are many considerations when building each of these surface models.
There are many considerations when building each of these surface models.
====Building the GR model====
====Building the GR model====
Once constructed, the base of volume model is subtracted from the top of volume model, creating the gross rock thickness model. This model is positive where thickness exists and either zero or negative where it does not. The negatives in this case are desired as they allow clear definition of the reservoir edge (zero line).
Once constructed, the base of volume model is subtracted from the top of volume model, creating the gross rock thickness model. This model is positive where thickness exists and either zero or negative where it does not. The negatives in this case are desired as they allow clear definition of the reservoir edge (zero line).
====Other variables for volumetrics====
====Other variables for volumetrics====